B rating for static content caching but no cache control headers sent?

On a Magento site, I get a B rating for caching static content. Mod_expires and Mod_headers are both enabled on the Apache server (2.4.7) running on Ubuntu 14.04. I am only using Magento's default .htaccess which has expires active on and a few directives for specific mime types.

However, the server doesn't seem to send any cache control headers. Here are the headers I see: